Image sensor arrays typically comprise a linear array of photosensors which raster scan an image bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors.
For high-performance image sensor arrays, one design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging generally without the use of reductive optics. In order to provide such a “full-width” array, however, relatively large silicon structures must be used to define the large number of photosensors. One technique to create such a large array is to make the array out of several butted silicon chips. In one proposed design, an array is intended to be made of 20 silicon chips, butted end-to-end, each chip having 248 active photosensors spaced at 400 photosensors per inch.
FIG. 2 is a schematic view showing a set of photosensors 10a–10z in a linear array, as would be found, for example, on a CMOS photosensitive device. The photosensors 10a–10z, which are typically in the form of photodiodes or photogates (depleted-gate photosensors), are operatively connected to a common video line 12, onto which each photosensor 10a–10z outputs a voltage representative of the light incident thereon at a particular time. As is known in the art such as in the patents incorporated by reference, each photosensor 10a–10z may further include, in addition to a photodiode, any number of ancillary devices, such as individual transfer circuits or amplifiers.
Each photosensor 10a–10z is connected to common video line 12 via an individual transistor switch, here shown as 14. The transistor switch 14 associated with the photosensor is independently controllable, for example, by application of a voltage to the gate of the transistor. Such a gate voltage closes the switch 14 so that a particular photosensor 10 may output a voltage signal onto the common video line 12 at the desired time for a coherent readout routine.
In order to read out the image signals from a sequence of photosensors 10a–10z in a manner convenient for image-processing apparatus, there is preferably associated with every transistor chip 14, a shift register, which comprises a set of what are known as “stages” 20. The stages 20 are arranged in series along a line 22, and are controllable via a pixel clock line 24.
According to a familiar method of operation of a shift register, each stage 20 along line 22 is capable of activating a particular transistor switch 14 associated with one photosensor 10a–10z. Ordinarily, each stage 20 “holds” a logical digital 0, unless and until there is entered into the particular stage 20 a digital 1, which is typically a one-cycle voltage pulse, along line 22. The single digital 1 is propagated along line 22, from one stage 20 to the next. When the 1 activates a particular stage 20, the associated transistor switch 14 is caused to make a connection between the associated photosensor 10 and the common video line 12. Operating the iteration of the digital 1 along line 22 is a pixel clock, in the form of a square wave of predetermined frequency apparent on line 24. This pixel clock signal ΦS activates one stage 20 along line 22 with every on-and-off cycle thereof. In this way, the photosensors 10a–10z are activated in a coherent sequence.
In practical applications of a system such as shown in the Figure, it has been found that the performance of the shift register and its associated circuitry may degrade significantly as the overall length of the device increases. Mainly, a long line for transmission of the pixel clock signal such as ΦS will have a large parasitic capacitance, as well a parasitic resistance, which will adversely affect the rise-fall time of signals on the line. The present invention is directed toward a design which circumvents this problem, so that very long devices, which can scan large hard-copy documents such as engineering drawings and posters, can be made.